/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
bool check(struct TreeNode* p, struct TreeNode* q) {
    if (q == NULL && q == NULL) {
        return true;
    }
    if (p == NULL || q == NULL) {
        return false;
    }
    if (p->val == q->val) {
        return check(p->left, q->right) && check(p->right, q->left);
    }
    else
        return false;
}
bool isSymmetric(struct TreeNode* root) {
    return check(root, root);
}